1. Field of the Invention
The present invention relates to a pointing device, and in particular, to a pointing device utilizing a finger-contact device to control a cursor movement on a computer display.
2. Description of the Prior Art
The term pointing device generally means a device that can input a cursor movement signal to a computer system, notebook computer (NB), or personal digital assistant (PDA) so as to execute a corresponding command. Examples of pointing devices include a computer mouse, trackball, touch pad, tablet, and joystick . . . etc. However, due to its convenience for a user's hand, the mouse is still supposed to be the most popular pointing device.
A computer mouse has two main types of sensing mechanism—a ball type (mechanical type) and an optical type. The ball type mouse uses a ball located thereunder for moving over a working surface, and a pair of shafts constantly in contact with the ball. The rotation of each shaft in response to ball movement causes generation of a corresponding cursor-moving signal and transmission of the cursor-moving signal to a computer system. The optical mouse, on the other hand, receives light reflected from a working surface to detect variations in an image position while moving over a working surface so as to generate a corresponding cursor-moving signal.
Still, either the ball type or the optical type computer mouse needs a large enough space to move over a working surface, which causes inconvenience if the space is limited. To solve this problem, a trackball has a ball which eliminates the need for a working surface. The user merely needs to use his finger to rotate the ball in order to generate a corresponding cursor movement signal. For example, U.S. Pat. No. 5,578,817 discloses a trackball device, in which the ball has been carefully designed with speckles, light spots, grids, or micro-texture, etc., to facilitate detection of ball rotation. However, this type of trackball has a high production cost.
In order to solve the problem of cost, another embodiment illustrated in FIG. 22 of U.S. Pat. No. 5,578,817 replaces the trackball with a fingerprint image detector. Furthermore, U.S. Pat. No. 5,623,553 also discloses a fingerprint image detector. Both of the cited references use a similar design including a light, a prism, a lens and a sensor, in which a light beam is projected onto a finger placed on the prism, and the fingerprint is reflected to the sensor so as to generate a cursor-moving signal. However, the cost of a prism is relatively high, and detection of the fingerprint has to be controlled quite precisely.
In addition, U.S. Pat. No. 5,801,681 discloses a method and apparatus for generating a control signal, where a light can project a light beam onto a platen such that a sensor can retrieve an image position according to the movement of a finger. However, when the finger is away from the platen but still in a very close position, the sensor can possibly retrieve a wrong image position and generate a wrong cursor-moving signal. Also, ambient light must be taken into account, and therefore the position of the sensor shown in FIGS. 2 and 5 of U.S. Pat. No. 5,801,681 to avoid sensing diffused or reflected light from beyond a critical angle, which is determined according to the material of the platen.